Known automatic transmissions for vehicle are disclosed in U.S. Pat. No. 7,828,688 (hereinafter referred to as Patent reference 1) and U.S. Pat. No. 7,131,926 (hereinafter referred to as Patent reference 2). The known automatic transmissions disclosed in Patent references 1 and 2 include three single pinion type planetary gear mechanisms and six engaging elements including two brakes and four clutches, and establish ten forward speeds and one reverse speed by engaging three elements among six engaging elements. As illustrated in FIG. 7, first, second, and third planetary gear mechanisms P11, P12, P13 are arranged in the mentioned order in a row from an input shaft N side to an output shaft T side. Elements that structure the first, second, and third planetary gear mechanisms P11, P12. P13 are referred to as first, second, and third carriers C11, C12, C13 that rotatably support first, second, and third pinions Q11, Q12, Q13, first, second, and third sun gears S11, S12, S13, and first, second, and third ring gears R11, R12, R13.
The second carrier C12 is connected to the input shaft N. The second ring gear R12 is connected to the third sun gear S13. The second sun gear S12 is selectively fixed to a housing H by a first brake B11 and is selectively connected to the first sun gear S11 by a second clutch CL12. The second carrier C12 is selectively connected to the first sun gear S11 by a first clutch CL11.
The second ring gear R12 is selectively connected to the first carrier C11 by a third clutch CL13. The first ring gear R11 is connected to the third carrier C13. The first carrier C11 is selectively connected to the third ring gear R13 by a fourth clutch CL14. The third ring gear R13 is selectively fixed to the housing H by a second brake B12. The third carrier C13 is connected to the output shaft T.
FIG. 8 shows operation states of clutches CL11, CL12, CL14 and brakes B11, B12, the operation states corresponding to each of speed stages. A circle marked in FIG. 8 indicates that the element is engaged. FIG. 9 shows a velocity diagram in which the sun gears S11, S12, S13, the carriers C11, C12, C13, and the ring gears R11, R12, R13 that are elements for structuring the first, second and third planetary gear mechanisms P11, P12, P13 when establishing a reverse speed, or reverse speed stage are arranged with intervals corresponding to gear ratios λ1, λ2, λ3 in a lateral axis direction, and rotation speed ratios corresponding to the elements are arranged in a vertical direction.
As shown in FIG. 9, according to an automatic transmission 12 for vehicle disclosed in Patent references 1 and 2, when establishing a reverse speed, the engaged state of the first clutch CL11 is established so that a rotation drive force of the input shaft N is inputted to the first sun gear S11. Further, drive force for reverse rotation is generated at the first ring gear R11 by connecting the first carrier C11 and the third ring gear R13 by establishing the engaged state of the fourth clutch CL14, and by fixing the first carrier C11 by establishing the engaged state of the second brake B12. The drive force for reverse rotation of the first ring gear R11 is outputted to the output shaft T via the third carrier C13. That is, according to the automatic transmission 12 for vehicle, the drive force for reverse rotation is generated only by the braking operation to the first planetary gear mechanism P11 and the input to the first planetary gear mechanism P11.
Then, because the third ring gear R13 is fixed and the third carrier C13 rotates in a reverse direction, the third sun gear S13 passively rotates in the reverse direction at speed faster than the third carrier C13. Further, because the second ring gear R12 is connected to the third sun gear S13, the second ring gear R12 rotates in the reverse direction. Because the rotation of the input shaft N is inputted to the second carrier C12, the second sun gear S12 comes to rotate at the high speed. That is, a rotation speed ratio of the second sun gear S12 corresponds to a value H that is calculated by adding an input rotation speed ratio to a value obtained by multiplying the reciprocal of the gear ratio λ2 (=the number of teeth of the second sun gear S12/the number of teeth of the second ring gear R12) and the sum of the absolute value of the rotation speed ratio of the second ring gear R12 and the input rotation speed ratio.
Thus, the second clutch CL12 that selectively engages the second sun gear S12 with the first sun gear S11 and the first brake B11 that selectively engages the second sun gear S12 to the housing H may be susceptible to the generation of the burning because a relative rotation speed is increased. Further, the durability of a bearing or the like that supports the second sun gear S12 declines significantly. In a case where an oil passage is formed at a shaft connected to the second sun gear S12, the durability of a sealing disposed on the shaft may be declined.
A need thus exists for an automatic transmission for a vehicle which is not susceptible to the drawback mentioned above.